4.2 years ago by
You should also read a few papers that discuss problems with t-tests and other statistics in Excel, for example Heiser 2006. Statistical tests, tests of significance, and tests of a hypothesis using Excel. Journal of Applied Statistical Methods, 5:155–171 or McCullough and Heiser 2008. On the accuracy of statistical procedures in Microsoft Excel 2007. Computational Statistics & Data Analysis 52:4570–4578. Here's a snippet from the second paper listing a few problems...
"Inaccurate t-test results in the presence of missing values. Does not correctly calculate t test values when there are missing data cells in the range. Microsoft identified this fault (KBA 829252) in previous Excel versions, but did not fix it.
Inaccurate p-values from a t-test. Excel uses the Welch test method, which calculates a non-integer degrees-of-freedom value. The Excel algorithm uses the TTEST function, which truncates the degrees-of-freedom input to an integer. This gives an incorrect p value for the test."